1. Field of the Invention
The field of the invention is generally electrical control circuits and in particular control circuits for a power supply.
2. Description of the Prior Art
There is an increasing usage of modular solid-state RF (radio frequency) power amplifiers in driver and output stages of pulsed radar transmitters. These RF amplifiers can be simply high frequency transistors. These transistors require input power at relatively low voltages of 25 to 50 volts but at relatively large currents. These requirements contrast with those for microwave tube transmitters which involve kilowatts of input voltage but at relatively low currents.
In order to minimize voltage droop at the input to these solid-state RF amplifiers a large amount of capacitance is usually placed locally in each RF amplifier module. It is these capacitors which provide power for the RF amplifiers during the short RF transmit pulse. The voltage on these capacitors will droop about one or two volts during the RF pulse. It is the function of the power supply to recharge these capacitors (during the radar inter-pulse period) back to their original voltage before the next RF pulse. The pulse-to-pulse tolerance on this voltage is determined by the radar performance requirements and the RF phase allocation of the radar system. Thus the power supply can be looked at as a capacitor charger and not as a normal DC supply.
In many applications, it is desirable to minimize the size and weight of the power supply. Therefore large and heavy 60 Hz transformers were displaced by off-line switching type power supplies. One such power supply is the series flyback stepcharger shown in schematic representation in FIG. 1. This power supply is the subject of a patent application of this inventor and other filed Aug. 11, 1982, Ser. No. 407,239, herein incorporated by reference. This power supply operates from a three phase, 60 Hz, 440 VAC.sub.L-L prime power system entering the power supply on the three AC lines 10,12, and 14. With such a power supply, the rectified input is 650 VDC which is beyond the 450 V limitations of V.sub.ceo, the collector-to-emitter breakdown voltage with open-circuited base, in presently available transistors. This problem is circumvented by putting in series three identical but separate flyback inverter switch sections 20, 22, and 24. The outputs of all the sections are combined through separate primaries or primary windings 26, 28 and 30 of the inverter output transformer 32, into a single secondary 34. The secondary 34 supplies current through a rectifying diode 36 to a load capacitor 38 which serves as the local DC power supply for a time varying load put across the leads of the load capacitor 38. The power supply serves to recharge the load capacitor 38 after it has been partially discharged by the pulsing of the RF amplifier.
The stepcharger drives the RF amplifiers directly with no series regulator disposed therebetween. The operation of the flyback stepcharger is controlled by six switching transistors 40, 42, 44, 46, 48 and 50, two per serial section 20, 22 or 24. Because of the series, none of the transistors 40-50 are exposed to the full 650 VDC.
The control circuitry required to control the switching transistors 40-50 presents several unique problems. Fine regulation is required to provide the pulse-to-pulse stability for radars, particularly those of the MTI type. Because the stepcharger operates as a flyback switch which induces current in the secondary when the primary current is turned off by the switching transistors 40-50, the secondary output current starts at a peak that may reach 200A before linearly decreasing to zero. Even a couple milliohm wire resistance to the load capacitor 38 introduces 400 mV variation in output voltage if measured close to the secondary. Regulation to 20 mV is difficult when a 400 mV ripple exists on the DC line.
It is desirable to let the secondary current decrease to zero before the switching transistors 40-50 are turned on for the next cycle. This procedure both reduces stress on the transistors and also maximizes the transfer of energy from the primary to the secondary. If the peak current on the primary is kept constant, then the discharge time of the secondary is inversely proportional to the voltage across the load capacitor. Thus the discharge time of the flyback stepcharger must change for at least three occasions when the output voltage is changed: (1) during initial power supply turn-on; (2) after an output overload when the load capacitor 38 is excessively discharged; and (3) when a different output voltage is desired. The last occasion arises when control of the output voltage is used to control the RF output power.